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January 25, 2013 11:42 AM Flu Shots Don’t Cause Fetal Death

By Aaron Carroll

A few years ago, when everyone was panicked about the swine flu, pregnant women appeared to be at significant risk to be seriously ill if they got the flu. But there were some anecdotal reports about fetal death in women who were vaccinated. Many people, including my friends, began to believe that the vaccine was dangerous. The NEJM is on the case, in “Risk of Fetal Death after Pandemic Influenza Virus Infection or Vaccination“:

METHODS: We explored the safety of influenza vaccination of pregnant women by linking Norwegian national registries and medical consultation data to determine influenza diagnosis, vaccination status, birth outcomes, and background information for pregnant women before, during, and after the pandemic. We used Cox regression models to estimate hazard ratios for fetal death, with the gestational day as the time metric and vaccination and pandemic exposure as time-dependent exposure variables.

Because those crazy European countries often have coordinated electronic medical data, they were able to study all 117,347 pregnancies in Norway in 2009 and 2010. Overall, fetal mortality was 4.9 deaths for every 1000 births. In the same time period, 54% of pregnant women were vaccinated in the second or third trimester. The first bit of good news is that the vaccine worked. Those who were vaccinated during pregnancy had a significantly lower chance of getting the H1N1 flu (hazard ratio 0.30). This is important, because in pregnant women who became infected with H1N1 influenza, fetal death was more likely (hazard ratio 1.91).

The risk of fetal death was not increased with vaccination. It was, in fact, lower in women who were vaccinated during pregnancy, although the result was not statistically significant (hazard ratio 0.88).

Bottom line: H1N1 influenza infection was associated with fetal death, not the vaccine. The vaccine prevented infection.

Get your flu shots, please.

[Cross-posted at The Incidental Economist]

Aaron Carroll ,MD, is an associate professor of Pediatrics and the associate director of Children’s Health Services Research at Indiana University School of Medicine.
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